1. Field of the Invention
The present invention relates to an image forming apparatus and image forming method wherein independent plural image forming units form primary images respectively and those primary images are superimposed on a medium to form a resultant image. More particularly, it relates to an image forming apparatus and image forming method that conducts positional correction with respect to superimposing position of each primary image.
2. Description of Related Art
There have been used tandem-type image forming apparatuses such as color printers, color copiers, and the like, wherein independent plural image forming sections for each color form their respective single color images and the single color images are superimposed on a carrying belt or on printing material mounted on the carrying belt. Tandem-type image forming apparatuses have an advantage such that output speed of a color image is fast in comparison with multi-cycle type ones in which each single color image is formed and superimposed at same image forming section in order. On the other hand, tandem-type ones need special device to secure superimposition accuracy among colors. That is, there occur uneven rotation among rotating members such as photosensitive drums of respective image forming section and uneven running of a carrying belt, due to degree of mechanical accuracy or the like. In case image formation is made without mending such uneven operations, images of respective colors do not meet one another and color shift occurs.
Image forming apparatuses of this type have conventionally been designed to execute a correction mode to correct such color shift. That is, under a correction mode, respective image forming sections form respective test patterns to meet positions of respective color images on a carrying belt. Positions of respective test pattern are read so as to grasp degree of positional shift of respective colors. Thereby, positional correction data of respective colors can be obtained for actual image formation. When actual image formation, images of respective colors are formed on positions corrected in accordance with positional correction data, whereby superimposition accuracy among respective color images is secured.
JP Laid-open Patent Publication No. 2000-194176 discloses an image forming apparatus that executes such correction mode. The image forming apparatus directed to No. 2000-194176 has two correction modes, namely, a first correction mode that gives preference to processing time required for correction, and a second correction mode that gives preference to correction accuracy. By selecting and using those two correction modes properly, the image forming apparatus intends to secure superimposition accuracy and prevent image formation speed from lowering. Furthermore, an image forming apparatus directed to JP Laid-open Patent Publication No. 8-305108 discloses system to detect internal temperature. That is, heat expansion occurring to each section due to internal temperature potentially causes positional shift among respective color images. Furthermore, in No. 8-305108, it is determined whether or not correction mode should be executed basing on internal temperature and past history. This mechanism intends to reduce execution frequency of correction mode that causes printer operation to stop.
However, those conventional image forming apparatuses have had the following problems. As to No. 2000-194176, correction accuracy is significantly low under first correction mode that gives preference to processing time required for correction. As to No. 8-305108, color shift correction could not be conducted appropriately under operational condition such that internal temperature changes continuously, e.g., during multi-printing operation or the like. In the event, multi-printing operation is sometimes forcedly interrupted due to correction mode execution. Therefore, actual productivity was poor.